Preliminary reports at about 2:50 a.m. were of a dark-colored sedan driving north in southbound lanes near Hillsdale Boulevard, Paulson said.

Another vehicle apparently hit the wrong-way driver and crashed into the center divider. A motorcycle was reported to have fallen over in the same area, but it is unknown if that was related to the crash, Paulson said.

At 3:51 a.m., several cars hoping to avoid congestion tried to turn around and drive north in the southbound lanes. The CHP quickly quelled the wrong-way traffic with assistance from San Mateo police, Paulson said.

A Sig-alert was issued at 3:09 a.m. and will remain in effect into the afternoon, according to the CHP.

The CHP is recommending that motorists heading south use Interstate Highway 280 and northbound drivers such as those heading to San Francisco International Airport take Highway 280 to northbound El Camino Real.